Using MicroStation since...
Read the What's new for V8i'. If you see a new widget you can't live without then yes it is worth the effort to upgrade to the current release of InRoads V8i.
With that said I would suggest you read the Whats new for the next release of V8i. The upgrades to MicroStation looks worthwhile all by themselvs. The enhancements to InRoads will be fantastic or fatal depending on how my users react to the changes. I hear the next version should be out the end of the year. I hope to upgrade to it in March(?). The enhancements will give the designer a great deal more power. Fun, fun!!
Is the only issue with working between InRoads V8i and InRoads XM the Geometry file (.alg)? Is this the only file a user would have to save back to the 8.9 version after opening and modifing in V8i? Any other issues with file between to two version of InRoads?